In rechargeable electrochemical cells, weight and portability are important considerations. It is also advantageous for rechargeable cells to have long operating lives without the necessity of periodic maintenance. Rechargeable electrochemical cells are used in numerous consumer devices such as calculators, portable radios, and cellular phones. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable electrochemical cells can also be configured as larger "cell packs" or "battery packs".
Rechargeable electrochemical cells may be classified as "nonaqueous" cells or "aqueous" cells. An example of a nonaqueous electrochemical cell is a lithium-ion cell which uses intercalation compounds for both anode and cathode, and a liquid organic or polymer electrolyte. Aqueous electrochemical cells may be classified as either "acidic" or "alkaline". An example of an acidic electrochemical cell is a lead-acid cell which uses lead dioxide as the active material of the positive electrode and metallic lead, in a high-surface area porous structure, as the negative active material. Examples of alkaline electrochemical cells are nickel cadmium cells (Ni-Cd) and nickel-metal hydride cells (Ni-MH). Ni-MH cells use negative electrodes having a hydrogen absorbing alloy as the active material. The hydrogen absorbing alloy is capable of the reversible electrochemical storage of hydrogen. Ni-MH cells typically use a positive electrode having nickel hydroxide as the active material. The negative and positive electrodes are spaced apart in an alkaline electrolyte such as potassium hydroxide.
Upon application of an electrical current across a Ni-MH cell, the hydrogen absorbing alloy active material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, forming a metal hydride. This is shown in reaction equation (1): ##EQU1##
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released from the metal hydride to form a water molecule and release an electron.
Hydrogen absorbing alloys called "Ovonic" alloys result from tailoring the local chemical order and local structural order by the incorporation of selected modifier elements into a host matrix.
Disordered hydrogen absorbing alloys have a substantially increased density of catalytically active sites and storage sites compared to single or multi-phase crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
Some extremely efficient electrochemical hydrogen storage alloys were formulated, based on the disordered materials described above. These are the Ti-V-Zr-Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 ("the '400 Patent") the disclosure of which is incorporated herein by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti-V-Ni composition, where at least Ti, V, and Ni are present and may be modified with Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain, but are not limited to, one or more phases with C.sub.14 and C.sub.15 type crystal structures.
Other Ti-V-Zr-Ni alloys, also used for rechargeable hydrogen storage negative electrodes, are described in U.S. Pat. No. 4,728,586 ("the '586 Patent"), the contents of which is incorporated herein by reference. The '586 Patent describes a specific sub-class of Ti-V-Ni-Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 Patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them. Other hydrogen absorbing alloy materials are discussed in U.S. Pat. Nos. 5,096,667, 5,135,589, 5,277,999, 5,238,756, 5,407,761,and 5,536,591,the contents of which are incorporated herein by reference.
The reactions that take place at the nickel hydroxide positive electrode of a Ni-MH cell are shown in reaction equation (2): ##EQU2##
Examples of nickel hydroxide active materials are discussed in detail in U.S. Pat. Nos. 5,344,728, 5,348,822, 5,637,423, 5,523,182, 5,569,563, the contents of which are incorporated by reference.
At present, sintered or pasted nickel hydroxide positive 25 electrodes are used in NiCd and Ni-MH cells. The process of making sintered electrodes is well known in the art. Sintered nickel electrodes consist of a porous nickel plaque of sintered high surface area nickel particles impregnated with nickel hydroxide active material either by chemical or electrochemical methods. To achieve higher discharge capacity and percent utilization, the trend has been away from sintered positive electrodes and toward foamed and pasted electrodes. Pasted nickel electrodes consist of nickel hydroxide particles in contact with a conductive network or substrate. Examples include plastic-bonded nickel electrodes using graphite as a microconductor, and foam-metal electrodes using high porosity nickel foam as a substrate loaded with nickel hydroxide particles. Pasted electrodes of the foam-metal type now dominate the consumer market due to their low cost, simple manufacturing, and higher energy density relative to sintered nickel electrodes.
The present invention discloses active compositions for the negative electrodes as well as for the positive electrodes of alkaline electrochemical cells. The active compositions provide for increased electrode capacity and utilization.